This antibody specific for ribosomal protein L9 was prepared by Dohn Glitz at UCLA. Anti L9 is a monoclonal antibody that belongs to subclass IgG 1. The specificity was confirmed by one and two dimensional gel electrophoresis followed by Western blot analysis. This antibody reacts strongly with protein L9 without any cross reactivity with any other 50S or 30S ribosomal proteins. The antibody recognizes both 50S subunits and intact 70S ribosomes. Our interest in localizing protein L9 on 50S ribosome is due to several interesting facts. Ribosomal protein L9 (MW 15,531) is known to have several functions. L9 is a primary RNA binding protein and it binds to the 3' end of the 12S fragment of 23S RNA which contains the sites associated with peptidyl transferase activity. Further, the binding of anti L9 to 23S RNA contains residues implicated in the ribosomal E site and the EF-G binding site. L9 has been crosslinked to proteins L1, L2, L5, L16 and L28 which are located at the peptidyltransferase center.